In recent years, there has been a dramatic increase in the demand for propylene to feed the growing markets for polypropylene, propylene oxide and acrylic acid. Currently, most of the propylene produced worldwide (74 million tons/year) is a by-product from steam cracking units (57%) which primarily produce ethylene, or a by-product from Fluid Catalytic Cracking (FCC) units (30%) which primarily produce gasoline. These processes cannot respond adequately to a rapid increase in propylene demand. However, the processing of lower cost butenes co-produced by these processes gives the refiner or the petrochemical producer an opportunity to add value depending on downstream integration and relative economics. The so-called on-purpose propylene processes such as propane dehydrogenation (PDH), metathesis of ethylene and butenes, high severity FCC, olefins cracking and methanol to olefins (MTO) contributes about 12% of total propylene production. However, propylene demand growth has exceeded ethylene and gasoline/distillate demand growth, and propylene supply has not kept pace with this increase in demand.
Olefin metathesis is considered a useful reaction to shift the composition of a pool of low-value butenes to meet market demand for propylene. In 2010, propylene production via metathesis accounted for about 5% of global propylene supply. This segment has been the most rapidly growing on-purpose propylene production route over the past 5 years. With new capacity coming on-stream in the Middle East and Asia, propylene production via metathesis is expected to increase.
Catalytic olefin metathesis is a useful chemical reaction that is able to transform simple and cheap organic molecules into complex and valuable molecules. In olefin metathesis, two olefin molecules exchange the groups around the double bonds in the presence of a catalyst. The olefins can be of different molecules by structure and composition, or two identical molecules. In general, reaction temperatures for olefin metathesis reactions can be as low as at room temperature or can be at temperatures up to about 500° C. or greater, depending on the type of starting materials, the catalyst used, and the media in which the reaction is carried out.
However, olefin metathesis catalysts often do not have the requisite selectivity to yield propylene and other products. Additionally, olefin metathesis catalysts are subject to deactivation due to coking from aromatic products.